President Barack Obama has secured 42 votes in the U.S. Senate for the nuclear deal with Iran, more than enough to keep Congress from passing a resolution rejecting the pact.

Democratic Senators Richard Blumenthal, Gary Peters, Ron Wyden, and Maria Cantwell announced on September 8 that they would support the agreement as the Senate moved to begin debate on the measure on September 9.

Forty-two votes is one more than the minimum needed in the 100-member Senate to block a resolution disapproving the nuclear deal, overcoming the unanimous support of Senate Republicans.

By using blocking tactics, Senate Democrats could spare Obama the embarrassment of having to use his veto power to protect a deal reached with five other world powers, which the White House sees as an important legacy and foreign policy achievement.

At the White House, Press Secretary Josh Earnest indicated the administration prefers not to have to veto the bill.

"If we have to go through the procedural charade of a veto, and a vote to sustain the veto, it will be embarrassing for this administration and this country," said Senator Chris Murphy, a Democrat who's been involved in coordinating support for the agreement.

"So I think it's cleaner, simpler, and much better for American credibility around the world if the motion to disapprove doesn't get past the Senate."

But Senate Majority Leader Mitch McConnell pleaded with colleagues not to block the disapproval measure.

"I call on every senator to resist attempts to obstruct a final vote and deny the American people and Congress the say they deserve on this important issue," McConnell said. "The Senate should not hide behind procedural obfuscation to shield the president."

As the Senate begins debating the measure, leading Republican presidential candidate Donald Trump is scheduled to headline a rally outside the Capitol building denouncing the deal.

The House will vote on a disapproval resolution later this week, and it is expected to pass there with unanimous Republican support.

Even Democrats who decided to vote for the deal remained skeptical about whether Iran can be trusted to do its part.

The agreement would provide Iran with hundreds of billions of dollars in relief from international sanctions in exchange for a decade of constraints on the country's nuclear program. It aims to keep Iran at least a year away from being able to produce enough nuclear material for a weapon.

"While this is not the agreement I would have accepted at the negotiating table, it is better than no deal at all," Blumenthal said.

Should Washington walk away from the deal that it negotiated along with Britain, China, France, Germany, Russia, and Iran, "the United States, instead of Iran, would be isolated," he said.

"This agreement with the duplicitous and untrustworthy Iranian regime falls short of what I had envisioned. However, I have decided the alternatives are even more dangerous," Wyden said.

Another Senate Democrat, Joe Manchin, announced he has decided to vote against the deal.

The last-minute statements of support came as a surprise as three of the Democrats had been considered possible "no" votes. It gave a dramatic start to what promises to be a bitter, partisan debate on the deal. Congress has until September 17 to complete action on the measure.